All about salt
Created: 13.4.24
Updated: 9.5.24
Context
Often when you hear "salt" you think of that delicious thing we add to foods: sodium chloride or table salt. However, salts are a general term that describe ionic compounds, such as potassium chloride, calcium carbonate, magnesium sulfate (or any sulfate for that instance), even bicarb soda. In this blog post, I will talk mostly of Sodium chloride (NaCl), however, sometimes when I mention "salt" I am talking about other ionic compounds, which I will try to make more explicit.
I have adapted to a vegetarian diet over the past few years. I enjoy it because it means that I need to be more creative with the meals that I prepare. Lately, I have taken an interest in preparing tofu dishes. For stir fries, I will just marinade in a sauce that I either found a recipe to or that I invented myself. For other dishes such as a "parmigiana", I will crumb the tofu and fry it. However, simply crumbing and frying tofu does not guarantee that it will be crispy.
A few days ago, I was in search of inspiration for a meal. Thats when I found a blog post for crispy tofu. She simply soaked the tofu in salt water and then fried it. This made so much sense to me as a chemist. She was basically "salting-out" the tofu. Therefore, I decided to try it out. I cut the tofu into small bite size pieces, and submerged in salt water (1 tsp in 2 cups of water). I left it to sit for 1 hour, then I drained the water and let the tofu dry. In the evening, I glazed the uncooked tofu with a small amount of vegetable oil and air fried for half an hour (rotated the tofu after 15 mins). It was the crispiest tofu that I had ever made. I was so excited and therefore, I felt the need to write a short article about salt.
First I will start with how we use it in chemistry. Then I will talk about how these concepts can be further used in cooking. Finally, I will talk about salt in the oceans and how it is connected to climate change.
Using salt in organic synthesis:
One of the fields that I am trained in is organic chemistry. A common washing technique that is used in organic synthesis is “salting out”. Salting out is a concept used to describe the washing or rinsing of an oil layer with saturated sodium chloride, or in other words, washing oil with highly concentrated salt water. In this instance, salt water is used to dry the organic layer (oil layer), as we want the compound that is dissolved in the organic layer to be as dry as possible to allow for easier purification.
As mentioned in solvation effects, sodium chloride is an ionic compound/salt consisting of Na+ and Cl-. The slight charges associated with water allow the salt to dissolve. However, there is a point that the salt dissolution becomes hindered. This is termed “saturated”. In a saturated salt solution, also known as brine, the salt molecules will try to pull water from the places that it is exposed to, as it prefers to be more dilute
Another reason for washing an organic solvent/mixture with brine is to decrease the solubility of the compound of interest and thus push it into the organic layer. This idea is represented through the Sechenov equation, which states that the salt (any salt) concentration is proportional to the salting-out effect.[1] Yet, there are some cases where the addition of salt (other than NaCl) to water may produce a salting-in effect. In this case, the salt usually consists of a higher molecular weight or an increase in the number of alkyl groups to render it “hydrotropic”: both organic and water soluble.[2]
Using salt for cooking:
Organic synthesis is very similar to cooking. I often say that organic chemists are just highly qualified chefs. Many principles of organic chemistry can be used in cooking.
A common strategy to render a more tender meat is to use salt. By salting a piece of meat or fish, salt will diffuse through the meat. The moisture on the inside will travel to the outside to try and maintain an equal amount of salt water on the inside and outside of the cell walls. Over time, as the salt travels further through the meat, the water will travel back through the meat.[3]
The right amount of time and right concentration of salt will allow for a juicy piece of meat. Dilute salt concentrations, produces a salting-in effect where proteins have a higher level of hydration. While concentrated salt will decrease the hydration of proteins. If salt is left long enough, the proteins will eventually denature. This is typically the science behind curing bacon and other cold meats. It is also the reason that cured products are sold in brine,[4] and explains why salt should never come in direct contact with yeast when preparing breads/doughs. The use of salt in doughs, however, allows the gluten to denature and exhibit better structural stability and higher level of toughness. From online reading, it seems that the recommended quantity of salt should be ~ 30 g/L of water and 1.75–2.25% of the flour content.
Salt in the ocean
The main ions present in our oceans are sodium and chloride. The dissolution of these ions in the oceans seem to have occurred between 80-98 million years ago, well after life emerged on Earth. Therefore, life emerged from fresh water, which may have had large amounts of dissolved sulfates and CO2. Oxygen was in existence when bacteria evolved to photosynthesise. From this point onward, fresh water could absorb oxygen, which seems to correlate with the Great Oxidation Event.
Perhaps salt began to dissolve in water around the time that life took to the land as increasing salinity of water decreases the concentration of gasses dissolved in water.[5]
Now if we have a decrease in the number of gasses that can be dissolved into the oceans, we will have an increase in the density of such gasses in our atmosphere. This is not a novel idea and in fact in 2022, Olson et al. modelled the effects of ocean salinity on the climate of Earth.[5] They found that with increasing salinity in the oceans there were warmer climates. With increasing salt concentrations, the freezing point of water is decreased and therefore less likely to form ice caps. As ice caps are important for the reflection of solar energy,[6] the disappearance over time (all ice caps are expected to be melted by the year 3000), will result in a higher level of solar energy being absorbed by the Earth and therefore a drastic increase in surface temperatures.
References
(1) Hermann, C.; Dewes, I.; Schumpe, A. Chem. Eng. Sci. 1995, 50 (10), 1673-1675.
(2) Sergeeva, V. F. Russian Chem. Rev. 1965, 34 (4), 309.
(3) This, H.. Kitchen Mysteries : Revealing the Science of Cooking; Columbia University Press, 2007.
(4) Sinha, R.; Khare, S. K. Front Microbiol. 2014, 5, 165.
(5) Olson et al. Geophys. Res. Lett. 2022, 49 (10), e2021GL095748.
(6) Jin et al. J. Geophys. Res.: Oceans 1994, 99 (C12), 25281-25294.